When I create a 50G zvol, it gets "volsize" 50G, and it gets "used" and
"refreservation" 51.6G

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I have some filesystems already in use, hosting VM's, and I'd like to mimic the
refreservation setting on the filesystem, as if I were smart enough from the
beginning to have used the zvol. So my question is ...
What's the extra 1.6G for?
And
If I have a filesystem holding a single VM with a single 2T disk, how large
should the refreservation be?
If it's a linear scale, it should be 2.064T refreservation.